The name is synonymous with stealing, cheating and stealthy evil. But the age-old battle between parasites and their hosts is one of the most powerful driving forces in evolution. Without its plunderers and freeloaders, life would simply not be the same.
From viruses to tapeworms, barnacles to birds, parasites are among the most successful organisms on the planet, taking merciless advantage of every known creature. Take the tapeworm. This streamlined parasite is little more than gonads and a head full of hooks, having dispensed with a gut in favour of bathing in the nutrient-rich depths of its host’s digestive system. In its average 18-year lifespan, a human tapeworm can generate 10 billion eggs.
Many parasites, such as the small liver fluke, have also mastered the art of manipulating their host’s behaviour. Ants whose brains are infected with a juvenile fluke feel compelled to climb to the tops of grass blades, where they are more likely to be eaten by the fluke’s ultimate host, a sheep.
“They are really disgusting, but man, are they good at what they do,” says Daniel Simberloff, an ecologist at the University of Tennessee and translator of the popular French text The Art of Being a Parasite. “Evolution is in large part probably driven by parasites. It is the main hypothesis for the continuation of sexual reproduction. How much more important can you get?”
The parasites that have had arguably the biggest effect on evolution are the smallest. Bacteria, protozoans and viruses can shape the evolution of their hosts because only the hardiest will survive infection. And humans are no exception: the genes for several inherited conditions protect against infectious disease when inherited in a single dose. For example, one copy of the gene for sickle cell anaemia protects against malaria. And it is still happening today. HIV and TB, for instance, are driving evolutionary change in parts of our genome, such as the immune-system genes (New Scientist, 22 November 2003, p 44).
Hosts can influence the evolution of their parasites too. For example, diseases which require human-to-human contact for transmission often evolve to be less deadly, ensuring a person will at least live long enough to pass it on.
Parasites can also drive the evolution at a more basic level. Parasitic lengths of DNA called transposons, which can cut and paste themselves all over the genome, can be transformed into new genes or encourage the mutation and shuffling of DNA that fuels genetic variation. They have even been implicated in the origins of sex, as they may have driven selection for cell fusion and gamete formation (see opposite).
Crocodiles with gleaming gums, coral reefs, orchids, fish with glow-in-the-dark lures, ants that farm, new directions for evolution. All that from swapping food – for cleaning services, for transport, for sunscreen, for shelter, and of course for other food.
Symbiosis has many definitions, but we’ll take it to mean two species engaging in physically intimate, mutually beneficial dependency, almost invariably involving food. Symbiosis has triggered seismic shifts in evolution, and evolution in turn continually spawns new symbiotic relationships.
Perhaps the most pivotal couplings were the ones that turbocharged complex, or eukaryotic, cells. Eukaryotes use specialised organelles such as mitochondria and chloroplasts to extract energy from food or sunlight. These organelles were originally simpler, prokaryotic cells that the eukaryotes engulfed in an eternal symbiotic embrace. Without them life’s key developments, such as increasing complexity and multicellular plants and animals, would not have happened. “There are only two things that matter in this world: respiration and photosynthesis. Eukaryotes didn’t figure out either by themselves, they borrowed them from prokaryotes through symbiosis,” says Geoff McFadden of the University of Melbourne, Australia.
Symbiosis has popped up so frequently during evolution that it is safe to say it’s the rule, not the exception. Angler fish in the deep ocean host bioluminescent bacteria in appendages that dangle over their mouths. Smaller fish lured by the light are easy prey. At the ocean surface, coral polyps provide homes for photosynthetic algae, and swap inorganic waste products for organic carbon compounds – one reason why nutrient-poor tropical waters can support so much life. The algae also produce a chemical that absorbs ultraviolet light and protects the coral.
More than 90 per cent of plant species are thought to engage in symbiotic couplings. Orchid seeds are little more than dust, containing next to no nutrients. To germinate and grow, they digest a fungus that infects the seed. “Birds and animals and insects that are adapted to pollination and seed disposal, these are some of the greatest symbioses. Without them we wouldn’t have most of our flowering plants,” says Ursula Munro, an ecologist at the University of Technology in Sydney, Australia.
“Without symbiosis we wouldn’t have most of our flowering plants”
Plovers pick leeches from crocodiles’ teeth, offering dental hygiene in return for food. Leafcutter ants use chopped-up leaves as a fertiliser for the fungus they grow in underground chambers. The ants cannot digest the leaves but the fungus that feeds on them produces a tasty meal of sugars and starch while breaking down the toxins in the leaves. And there is not an animal out there, including us, that can survive without the bacteria that live in its gut, digesting food and producing vitamins.